Aqueous acidic solutions which are useful in forming phosphate coatings on zinc surfaces are well known. Certain of these solutions have achieved widespread commercial use. Such solutions typically include the phosphate ion, the zinc or manganese ion and typically one or more of the following ions: nickel, cobalt, copper, nitrate, nitrite, fluoroborate or silicofluoride. The art has been able to form phosphate coatings on zinc since about 1917, and there have been successive discoveries of the effects of the nitrate, copper, nickel, fluoborate, and silicofluoride ions on the coating ability of such solutions made through the years. Presently, galvanized metal surfaces are effectively provided with a deformation or paint base protective phosphate coating by being treated in the following manner: (1) pre-treatment steps which generally include a rinsing or cleaning step and an activation step; (2) a phosphate coating step; and (3) post-treatment steps including a general rinse step and a sealing rinse step. Such processes and solutions for forming conversion coatings on metal surfaces are well known and have been described, for example, in Metal Handbook, Volume II, 8th Edition, pages 529-547 of the American Society for Metals and in Metal Finishing Guidebook and Directory, pages 590-603 (1972), the contents of both of which are specifically incorporated herein by reference. Despite the advances, the best present day formulations are troublesome in certain respects.
For example, certain types of paint applied over the art-disclosed coatings develop a roughness which is referred to as hazing, and the gloss is not of the highest order. Moreover, painted surfaces subjected to bending do not resist flaking, cracking and the like to the commercially desired degree.
Another problem associated with such coatings, particularly when they are deposited on galvanized surfaces, is known as "white speaking" or "nubbing". This phenomenon can best be described as uncontrolled crystal growth at pinpoint locations. This growth results in a flawed, rough surface. The specks appear as large white growths; they are generally zinc or zinc/iron phosphate crystals. While they can vary greatly in size, they are typically 50-150.mu. wide and 100-400.mu. high.
The larger crystal growths are apparent to the naked eye from virtually any angle. The smaller growths can only be seen with some magnification. However, when the treated or coated metal surface is painted, such flaws are immediately apparent and the resulting product is frequently unacceptable. A uniform paint film cannot be applied; this is true whether the paint is applied by spray or electrodeposition. The "white specking" has been observed to occur during both the pre-treatment and treatment stages. However, it most commonly appears during the treatment stage.
There have been many attempts to effectively solve this problem of coating zinc surfaces, particularly galvanized surfaces; all such attempts have focused on the treatment steps. Two of the most effective are disclosed in U.S. Pat. No. 3,240,633, issued Mar. 15, 1966, to Gowman, et al., and U.S. Pat. No. 2,835,617, issued May 20, 1958, to Maurer; both of which are expressly incorporated herein by reference. These methods primarily involve the introduction of fluoride and ferric ion into the bath.
These references clearly suggest that the use of fluoride ion in the treatment bath assists in preventing the formation of "white specks" or "nubbing" under most conditions. However, it has also been observed that this method is not always effective in completely preventing the abnormal crystal growth, particularly when economical and environmentally sound levels of fluoride are used; the inclusion of additional fluoride being undesirable from both an economic and environmental standpoint.
It has now been discovered that while fluoride ion introduction has been viewed as usually effective in preventing the abnormal crystal growth, it is in fact the presence of chloride ion which causes the "white specking" or "nubbing". Further, an increase in chloride ion without a corresponding increase in fluoride ion will increase the frequency and severity of white specking. It will be appreciated that the art heretofore was wholly indiscriminate with regard to chloride level and the ratio of chloride to fluoride.
It is commercially impractical if not impossible to remove all chloride ions from such processes. This is a result of the many sources of such ions, including, for example: reaction products of chlorate accelerators, and other impurities; intentionally added salts such as the ferric chloride as suggested in U.S. Pat. No. 3,240,633 (discussed above); inert sodium chloride and other similar bulking or anti-caking agents and additives; make-up water, and the like.
Accordingly, the present invention relates to a method for providing an improved phosphate coating on zinc surfaces. The improvement comprises employing solutions and baths with a select fluoride:chloride ratio.